Self-assembled monolayers beyond thiols : dithiocarbamates - from pure layers to ternary assembly systems

摘要

Since microelectronics started to play its dominant role in the innovation ofudtechnologic and electronic instruments the reduction of the size of transistors andudother functional devices was in the main focus of research and development.udTherefore molecular nanotechnology and molecular electronics attracted a greatuddeal of attention in the last years. The understanding of the fundamentals ofudelectron transport through molecules is essential for the development andudexploration of possible electronic components. Many different approaches have beenudintroduced so far to characterise metal-molecule-metal junctions. The most reliableudmeasurements on single molecular junctions were done on break junctions, metaludgaps, which have been produced by the combination of metal microfabrication andudmetal electromigration as well as on the Scanning Tunneling Microscope. Theudmolecules under investigation were almost exclusively thiol functionalised. Auddetailed understanding of a molecule and how it affects the transport properties of audmetallic nano-gap must include the exact gap structure, the molecular backbone andudthe interfacial structure of the chemical/physical linker of the molecule to the metaludcontacts. The aim of this work was to investigate the electron transport abilities ofudmolecules – containing dithiocarbamate (DTC) as the linker group – on the singleudmolecular level.udTherefore the chemisorption process was investigated for different aromatic and nonudaromatic thiol and DTC compounds by Scanning Tunneling Microscopy, X-RayudPhotoelectron Spectroscopy (XPS) and Cyclic Voltammetry (CV). The obtainedudresults were published under the title: Dithiocarbamates: Functional and VersatileudLinkers for the Formation of Self-Assembled Monolayers. The most importantudfindings presented in that publication were that the dithiocarbamates as sodium saltsudand also the oxidized form of dithiocarbamates the thiurams are chemisorbing in audbidentate structure on Au(111). The thiurams are decomposing at the disulfideudbridge as the sodium dithiocarbamate releases the cation. Reductive desorptionudexperiments were accomplished in a home built electrochemical set-up, whichudallowed for the investigation of the same samples, investigated by XPS and STMudbefore. Comparing all these data quantitatively it was shown that the reductiveuddesorption is a one electron transfer process.ududA second publication was written on the novel and very interesting surface structureudof diethyldithiocarbamate: Supramolecular surface assembly of dithiocarbamates onudAu(111): Chiral trimeric domains and amorphous structures. Here we report on audfirst time observation that chemisorbing molecules are forming trimeric complexes,udwhich are hexagonally packed. This is the first experimental evidence of audchemisorbing supramolecular surface assembly.udThe next experiments were designed in order to produce STM accessible molecularudsurface structures i.e. islands of one molecular species placed within a matrix ofudanother thiol compound. Therefore we investigated the formation of mixedudmonolayers at the solid liquid interface of a non-polar and a polar thiol species.udThese results will be published as: Co-assembly – concentration and mixinguddependent structures: Mechanisms of molecular 2D ordering investigated byudNEXAFS and STM. The most important result from this work is a detailed model,udwhich can explain all the observed effects as there are ostwald ripening of vacancyudislands, the existence of a minimal stable island size of the low dosed species and theuddiffusion limits of thiol adsorption.udThe formation of ternary assemblies comprising dithiocarbamate and thiol linkedudmolecules within an alkanethiols self-assembled monolayer matrix made the firstudcomparative dithiocarbamate/thiol STM study possible. A gel-assisted assemblyudtechnique enabled us to produce molecular gradient-assemblies. The combination ofudNEXAFS and STM lead to the identification of the molecules bound to the surface.udFinally the STM data analysis lead to the first experimental evidence on the singleudmolecular level, that dithiocarbamate is a more conductive linker than a thiolateudlinker. The results of these investigations were submitted under the title: Gelassistedudhost/guest assembly of dithiocarbamates and thiols: Analysis of localudorientation and conductivity within ternary assemblies.